Transmission regulation



April 5', 1927. 1,623,600

.1. s. JAMMER TRANSMISS ION REGULATION Filed Nov`.26. 1924 2 Sheets-Sheet 1 DAz manna,

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Jacob J? Jammer i 1623 600 Alm! 5 1927 J. s. 'JAMMER TRANSMISSION REGULATION Filed Nov.2e. 1924 2 sheets-sheet 2 L L N E: I I l LLI .f

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.fdc Jammer by 477i! Patented pr. 5, i923".

JACOB S. JAMLER, OF NEW' YORK, N. Y., ASSIGNOR TO RVi-STERN ELECTRC COIIIPANY, INCORPORATED, GF NEXV YORK., N. Y., A CORPORATION OF NEVI YORK.

TRANSMISSION REGULATQN.

Application filed November This invention relates to transmission regulation and is particularly adapted for use in repeatere'd transmission systems.

An object of the invention is to stabilize the transmission characteristics in such systems.

A related object of the invention is to facilitate the maintenance ot a transmission regulating system.

Another object ol the invention is to provide a new and improved system tor automatically adjusting the gain and level of each of a plurality of repeaters in a carrier current transmission system.

In long line transmission systems and repeatered radio systems, it is desirable to maintain the output of each repeater at the maximum operating level in order that the signal currents, which are greatly attenuated duringl transmission, may be lrept as lar as possible. above the noise level on the transmission line or other medium.

rt the same time it is necessary to adjust the repeater gains to compensate :tor changes in the transmission characteristics of the transmission line in order to maintain a constant overall equivalent. Otherwise, changes in the attenuation of a transmission line, due tor example to varying' weather conditions., would result in large -fluctuations in the volume of a telephone or other message being transmitted.

Various means have been proposed for maintaining the output of such repeaters constant under varying conditions, such as measuring, indicating or controlling systems. It is well known, for example, that a pilot channel may be set asioe in a multiplex signaling system, or a pilot circuit or some other arrangement may be used in a transmission system, for the purpose ot automatically adjusting the repeater gains to maintain them at the proper level.

ln systems of this type, a pilot current is usually impressed upon the transmission line at one terminal for transmission to another terminal geographically remote theretron. At the intermediate repeater stations, the amplitude of the pilot current is used to regulate the repeater This is accomplished by connecting' a detector to the output ot' each repeater, this detector being mechanically connected to a gain control potentiometer in the repeater' circuit so that ze, i924. ser-m1 no. 752,319.

a change in the repeater output produces a compensating change in the repeater' gain to restore the output level to normal.

Certain diliiculties may arise, however, in gain control systems of the above type. For example, in signaling' system having four repeaters, when the line section between a transmitting terminal and the repeater station nearest thereto increases in attenuation by say two miles, innnediately the gain eontrol mechanisms at every repeater on the line respond to the decrease in level and each repeater is increased in gain by two miles. This results in a net decrease in overall transmission equivalent of six miles, that is, a two mile loss has been introduced upon the line and an attempt has been made to compensate for this loss by introducing an eight mile repeater gain. Since there has been no change in the attenuation of any of the line sections except the tirst line section, the repeaters associated with these other line sections would be operating at levels above the normal and accordingly they immediately begin to step the gain down, resulting in a swing in the opposite direction. This hunting action will continue until each repeater in succession has reached the normal level.

In order to improve upon the operation of systems of this tipe, it has been proposed to break up the gain control circuits into short lengths in such a way that each repeater is responsible tor only the preceding adjacent line section. rlhis is accomplished through the use of separate pilot oscillators at each repeater for supplying` pilot current to the. next adjacent line section. In this way t-he gain control mechanisms at the various repeater stations will be unadected by attenuation changes in a remote line section which are intended to be compensated for at a preceding` repeater station. Under certain circumstances, however, the attenuation o't a given line section may vary to such an extent that the associated repeater is unable to bringr the output level back to normal thus resulting in tluctations in the volume or" the message being transmitted.

This invention provides a system of transmission control for automatically regulating the transmission at each of a plurality of repeater points. Control mechanisms are provided at each repeater which are affected only by line changes in the preceding line section unless such line changes are greater than a single repeater can accommodate, in Which case the next repeater on the line is called upon to malte an adjustment in order to maintain a constant overall equivalent.

The invention ivill be described as applied to a multiplex carrier current telephone system, although it ivill be understood that it may also be applied to other signaling systems.

The various features and advantages of the invention will appear from the followiing description and the accompanying drawings in Which:

Fig. l is a schematic view illustrating the invention as applied to a plurality of repeater stations of a carrier current telephone system.

Fig. 2 is a diagrammatic view illustrating a terminal station oi a carrier suppression telephone system embodying the invention.

Fig. 3 is a diagrammatic view of a carrier telephone repeater embodying the invention.

Reference will rst be made to Fig. 1 Which shows a carrier current telephone system having West and east terminal stations and a plurality ot intermediate iepeaterstations R1, R2, and R4. In practice, it is often customary to provide a repeater at the terminal stations as well as at the intermediate repeaters, the gain ot' the terminal repeaters being one-half as great as that ot the intermediate repeaters.

The system of Fig. l diagraiiimatically illustrates the pilot control mechanisms for adjusting the gains and levels of the repeaters employed for transmission from the West terminal to the east terminal. A similar arrangen'ient ot' control mechanisms is employed for adjusting corresponding mechanisms at the repeaters for transmission from the east terminal to the west terminal.

At the West terminal, in addition to the usual signaling channel apparatus employed for the transmission of telephone messages, a pilot oscillator OW is associated with the transmission line ttor the purpose of transmitting a pilot current ot predetern'iined frequency to the repeater' R1. A pilot detector D, is associated with the output circuit of the repeater R, and is adapted to control the gain potentiometer P, in the repeater input circuit in accordance with the amplitude of the received pilot current, as will be hereinafter more fully described. The pilot current from oscillator OW, however, is prevented troni being transmitted to rcpeater R2 by means of suitable Wave filters, not shown. An independent pilot oscillator Ol is provided at repeater R1 for the purpose of transi'nitting a pilot current to rcpeater R2.

The mechanisms at all of the repeater stations are identical, a pilot oscillator O2 being provided at repeater R2 for the purpose of transmitting a pilot current to repeater and a pilot oscillator O., being provided at repeater R to transmit a pilot current to repeater R4, etc. At the east terminal, a detector DO in the output of the receiving amplifier or repeater serves to control the gain jiotcntiometer Pe thereat.

The pilot detectors at the various repeater stations, in addition to controlling their respective gain potentioineters also serve to control the out iut of their respective pilot oscillators O1, 2, 0:, and OA1 in accordance with the amplitude ot the received pilot current. ln this way the control mechanisms at each repeater are affected only by line changes in the preceding adjacent line section unless such changes are greater than a single repeater can accommodate. In the event that this latter condition should prevail, the pilot oscillator at the repeater in the atlected. line section will automatically have its output adjusted by an amount dependent on the difference in the level obtained and the level required so that the n uit repeater on the line is called upon to malte an adjustment in response to the amplitu de ot` the received pilot current in order to maintain a constant overall equivalent.

Normally, it' a. change in attenuation occurs in the line section between the West terminal and repeater Rl, only repeater R, Will malte a gain adjustnient. In the saine way, if a change in attenuation occurs in the line section between repeaters R1 and R2, only repeater will make the gain adjustment, etc. Under certain conditions, however, the attenuation ot' any line section, ior example the line section between the west terminal and repeater R may increase so much that the repeater Rl is unable to bring the output level back to normal. In this case, the detector D1 after increasing the gain ot repeater R to a maximum degree, then changes the output of pilot oscillator O, by an amount dependent on the difference in the output level obtained and the output level required. Thereupon, the control ii'iechanisins at repeater R2 in response to the change in amplitude in the received pilot current will increase the gain o't repeater R2 by the amount necessary to malte up for the lach oi gain in repeater R,. It' repeater R2 is unable to make up the total gain required, the output of pilot oscillator O2 is automatically adjusted to cause repeater R, to increase its gain, etc. In this Way a portion of the loss originally introduced on the line section between the west terminal and repeater R, is shifted successively from one repeater to another until it is entirely compensated for by the total gain oi the repeaters. It the attenuation ot' any line section decreases, the gain potentiometers at the various repeaters Will be adjusted to introduce a compensating loss into the circuit to maintain a constant overall equivalent.

The invent-ion is illustrated in detail in Figs. v2 and 3, which, when placed end to end with Fig. 2 at the lett, represent the west terminal station and a mid-line repeater of a carrier telephone system interconnected by the multiplex transmission line ML.

The usual east terminal station, which is identical to the west terminal station shown in F ig. 2, is omitted for the sake ot simplicity in the description. It will also be understood that although only one repeater station is illustrated in detail, a plurality7 ot other repeater stations are also connected to the line in a manner similar to that illustrated in Fig. 1.

The terminal apparatus at the west station comprises a plurality of transmitting channels TCl and TC2 connected through a common transmitting circuit TL to the main line ML, and a plurality ot receiving channels RC1 and RC2 connected through a common receiving' circuit RL to the main line HL.

Carrier currents are utilized tor transmission over the line ML and are grouped to their frequencies, the higher frequencies, as a group, being used tor transmission from west to east, and the lower frequencies, a group, being used for transmission from east to west.

A high pass grouping filter HP is paired with a low pass grouping filter LP at the west station. These .filters serve to separate the directional groups ot carrier waves to the respective terminal transmitting and receiving circuits. These tilters and each ot the other lilters shown throughout the system may be designed in accordance with the principles set forth in the United States patent to Campbell, No. 1,227,113, issued May 22, 1917.

Low frequency lines Ll and L2, which may be telephone lines or other types ot signaling lines, are associated with channels TCI-RC1 and TCg-RCg, respectively, for siniultaneous and independentcon'ununication over the line ML with corresponding similar lines at the east station (not shown).

rThe low frequency lines are provided with balancing artilicial lines or networks N, and N2, respectively, and with ditterential repeating coils H1 and H2, commonly known as hybrid coils, for enabling independent transmission in the two directions between the line and the high frequency terminal apparatus.

lransmitting channel TC1 includes a modulator M, and a band filter TBFV Similarly, transmitting channel l`C2 includes a modulator M2 and a band filter TBFT Receiving channel RCl includes a band lilter BBF1 and a demodulator and amplifier DA1, while channel RC2 includes a hand iilter BBF2 and a demodulator and amplitier DA2.

The modulators associated with the various transmitting channels throughout the system are ot the balanced type disclosed in the United States patent to Carson, No. 1,33,806, issued June 15, 192()` designed to suppress the unmodulated carrier trom transmission. r)The various demodulators and ampliliers may be ot the well known vacuum tube detector type shown in Fig. fI-S) ot' an article entitled Carrier current telephony and telegraphy by Messrs. Colpitts and Blackwell, published in the rl"i"ansactions et the American Institute ot' Electrical Engineers, Vol. 40, 1921.

The carrier system outlined above is, as stated, ot the type in which the carrier wave ot cach channel suppressed trom trans mission when no signals are .icing sent, and when signals are being transmitted modulation components ot the modulated wave are sent over the line, but no unmodulated components el the carrier wave are transmitted. ln order to reproduce the signal. trom modulated waves ot this character, it is necessary that the carrier wave supplied to the deniodulator at the receiver be ot' substantially the same itrequency as the carrier wave which was suppressed at the transn'iitter. jr system ot this general character is described in connection with Fig. l) ot' the Colpitts and Blackwell article, supra.

Carrier currents ot' the 'frequency assigned to channel TC, are supplied troni a source TO, to the modulator M, wherein they are modulated by voice frequency currents or other signaling currents trom trie low 'frequency line LJ.. Carrier source TO: is similarly associated with modulator HL., while carrier current sources RC, and lt( 2 are associated with the demodulators and amplifiers DA1 and DAL. respectively.

jrs noted above, the carrier current sources RC1 and R02 are necessary to supply to the incoming side bands in each receiving channel. the carrier current which was suppressed at the transmitting end ot the channel.

The various band i'ilters associated with the terminal transmitting` and receiving channels are so designed that they will trans mit one ot the side bands, either the upper or lower as desired, produced byv modulation in the respective channels and will suppress trom transmission frequencies lying outside ot such band.

Voice frequency currents originating, for instance, in the low frequency line Ll at the west station pass through the associated hybrid coil H, into the modulator M1 in the transmitting channel TC1. Ther-e is like` wise t'ed into the modulator the carrier current trom the oscillator T01, as described above. 'Of the `components of modulation appearing in the output circuit of the modulator M1, the transi'i'iitting band filter TBFL suppresses all except one side band, for example the upper side band, which it transmits or passes into the common transmitting circuit TL. The currents so transmitted from channel TC, and the other transmitting channels then ass through the high pass grouping filter HP to the multiplex line ML and thence through the repeater station to the east terminal where they are picked up by the high pass grouping filter thereat and are thence transmitted to the respective receiving channels.

Currents incoming at the west station pass through the low pass grouping filter LP, through a gain control potentiometer hereinafter to be described, to the common receiving circuit RL whence they are selected by the respective receiving band filters for transmission to the respective low frequency lines. Currents passing through receiving band filter RBF1, for example, are combined in the demodulator and an'iplifier DA, with carrier waves from source R01, and the voice frequency components of den'iodnlation appearing in receiving channel RC, are transmitted through hybrid coil H1 to the low frequency line L1.

In addition to the signaling channels indicated, a control or pilot channel is also associated with the line ML at the west station. The pilot transmitting channel comprises a pilot oscillator PO connected to the line ML through the pilot band filter PF1. The pilot oscillator PO is adapted to transmit a pilot current of a frequency different from that of any of' the signaling channels, but within the upper group of frequencies transmitted by the high pass terminal filter HP.

The pilot receiving channel at 'the west station is connected to the line ML through the low pass grouping filter LP, variable potentiometer and the pilot receiving band filter PF2 selective of the pilot frequency.

The pilot receiving channel may be of' the type disclosed in U. S. Patent 1,579,708 to Jammer et al., April 6, 1926, and includes an automatic controlling device 6 for controlling the adjustment of the potentiometer 5, a rectifier and amplifier 7 and an associated rectifier vbridge RB.

The potentiometer 5 is arranged to be reg ulated automatically in accordance with the amplitude of the received pilotcurrent to maintain the current impressed on the carrier receiving channels at a constant level. For this purpose the automatic controller 6 is arranged to move the variable taps of the potentiometer in response to slight variations in the received pilot current, the direction of adjustment of 'the potentiometer being always such as to compensa-te in the carrier receiving channels for variations in the received signaling currents.

The controller 6 may be of the type disclosed in the United States Patent to Brewer, No. 1,356,804, issued October 26, 1920, or any other suitable type of' automatic controller.

The rectifier 7 is associated with the receiving pilot filter PF2 through a transformer 8. The rectifier bridge circuit, which is described in detail in the above mentioned patent of Jammer et al., comprises resistance elements 9, 10, 11 and 12. Thebridge is adjusted in accordance with the principles set forth in the Jammer et al. patent so that an unbalanced current flows in the resistance 11, and is independent of the volt-l age fiuctuations of the rectifier plate battery 13 Within the desired operating limits. Due to this current, a steady drop of potential exists across resistance 11 except as it is altered by incoming variations from the line. The battery 13 is usually the plant battery at the local telephone exchange, or may be the plant battery for Morse telegraph lines. As it becomes discharged and as it is recharged its voltage changes. Due to fthe rectifier bridge circuit arrangement, however', these voltage variations have no effect upon the controlling and indicating circuit.

A galvanometer circuit is associated with the rectifier bridge and includes the galvanometer 14, an adjustableportion of resistance 11, an adjustable portion of resistance 15,'variable resistance 16 and a source 17 of constant potential The galvanometer is adapted to control the operation of the antomatic controller 6.

The potentiometer control shaft 18 is operated by the motor 19 and is suitably geared to the variable taps on the potentiometer .5 so as to vary the voltage applied to the common receiving circuit RL.

Normally, resistance elements 11, and 16 are so adjusted that the currents in 'the differential windings of the galvanometer 14 are exactly equal under predetermined conditions of transmission on the line ML so that the galvanometer 14 is maintained in its zero or mid-position. As 'the attenuation of line ML departs from normal. however, corresponding variations occur in the strength of the received current. These current changes appear in amplified form in the resistance 11, causing the galvanometer 14 to deflect and to effect such an adjustment of potentiometer 5 as will restore the current in branch 11 to its normal value. The ad justment of the taps associated with the resis'tance elements of Apotentiometer 5 introduces a compensating gain or loss. as the case may be, in the common receiving circuit RL, thus maintaining the received currents at a constant level..

The cont-rol apparatus 6 constructed in ac- CII cordance with the Brewer patent, supra, not only serves to adjust the potentiometer 5 to compensate for variations in the received currents, but also is provided with a recording chart and associated mechanisms for making a permanent record of the variations produced by the deflect-ion of the galvanometer 14.

Fig. 3 shows in detail the circuits eniployed at each of the repeater stations associated with the transmission line.

As pointed out above, the currents transmitted from the west station, which are impressed upon the repeater from line section W, are comprised in a higher frequency group than those transmitted from the east station which are impressed upon the repeater from line section E.

The repeater consists of upper and lower repeater branches which include one-way selective paths for each of the frequency groups. rhese selective paths include .fre quency discriminating filters which direct the incoming` currents through the proper repeater channels.

The upper branch of the repeater -includes a high pass input filter HPlF designed to pass frequencies of the upper group transmitted from west to east, an attenuation equalizer AE, gain control potentiometer 20, an amplifier WA and a high pass output filter HPOF. The transmission frequency range of the filter HPIF at each repeater station is sufficiently wide to include the frequency of the pilot generator whilethe range of the filter HPOF is not sufficient to pass the pilot frequency.

The lower branch of the repeater, as shown, contains a low pass input filter LPIF designed to pass frequencies of the lower group transmitted from .east to west, an attenuation equalizer AE, an amplifier EA and a low pass output filter LPOF.

The attenuation equalizers AE may be of the type disclosed in the United States pat.- eiit to Raibourn, No. 1,413,357, issued April 18, 1922, and serve to equalize the differences of line attenuation at the different carrier frequencies used, thus giving outputs from the rep-eaters of substantially equal value for the several frequency channels regardless of the line attenuation.

For the sake of clarity and simplicity in the description, the transmission regulating mechanisms of the invention `are illustrated only in connection with the upper repeater branch for controlling the transmission of carrier curr-ents transmitted from west to east. It will be understood, however, that similar regulating circuits are also to be employed in connection with the lower repeater branch at each repeater station for controle ling the transmission of carrier currents from east to west.

, shaft 22,

At the repeater, carrier currents comprised in the upper group of frequencies incoming from line section XV pass through the high pass input lter HPIF, attenuation equalizer AE, potentiometer 20, are amplified in amplifier WA and then passed through the high pass output filter HPOF to line section E. Carrier currents comprised in the lower group of frequencies incoming from lin-e section E pass through the low pass input filter LPIF, attenuation equalizer AE, a potentiometer (not shown) similar to poteiitionieteil 20, are amplified in amplifier EA and then transmitted through the low pass output filter LPF to line section W.

The gain control mechanisms at the repeater station include the potentiometers 2O and 21 mounted on a common controlling a galvanoineter 23 and an automatic controlling device 24 operated thereby.

The incoming pilot current in the upper branch of the repeater passes through the potentiometer 20 and` amplifier WA and is selectively transmitted by a pilot filter PF3 to the output of which is coupled the rectifier bridge circuit 25. The received pilot current, however, is prevented from being transmitted to line section E by the high pass output filter HPOF which suppresses currents of this pilot frequency.

A pilot oscillator POl is provided at the repeater for the purpose of transmitting a pilot current to the succeeding line section E. This pilot oscillator is connected to line section E through the potentiometer 21 and pilot lter PF4. A rectifier bridge circuit 26 is bridged across the output circuit of the pilot oscillator PO, between the potentiometer 21 and the pilot filter PF4.

The rectifier bridge circuits 25 and 26 are similar to the rectifier' bridge circuit described in connection with Fig. 2, the rectified current in the rectifier bridge circuit 25 setting up a direct current potential in an adjustable portion of resistance arm 27, .and the rectified current in the rectifier bridge circuit 26 setting up a direct current potential in an adjustable portion of resistance arm 28. The potential set up in resistance 27 of rectifier bridge 25 is balanced against the potential set up in resistance 28 of rectifier bridge 26 in the differential galvanometer 23. The rectifier bridge circuits may be ad- I just-ed so that the potential set up in resistance 28 is exactly equal to the potential set up in resistance 27 when the amplifier WA is operating at its normal output level and the pilot oscillator PO1 is passing the normal amount of pilot curr-ent to line section E. However, when the potential in resistance 27 d-eparts from normal, due to changes in the amplitud-e of the received pilot current, the galvanoineter balance will be upset and the galvanometer 23 will be deflected to effect such an adjustment of potentiometer 20 as will restore the current in resistance 27 t0 its normal value.

In describing the operation ot' the transmission regulating system at the repeater, let it be assumed that the line ction lV increases in attenuation so that the output level of amplilier @VA is correspondingly decreased. Under such conditions the pilot current incoming` from line section lV is attenuated more than the normal amount and hence the direct current potential set up in resistance 2T is less than the direcl current potential set up in resistance 2S, since the pilot oscillator P0l is still operating at its normal output level. This condition upsets the voltage balance on the galva-noineter 23 and causes the automatic controller 24 to set the adjusting mechanisms in motion, rotating the potentiometer shalt in a counter-clockwise direction. As the shaft 22 rotates in a counter-clockwise direction the effective resistance introduced in the upper branch ol' the repeater by the potentiometer is increased, thus increasing the voltage drop in this circuit and correspond ingly increasing the gain oit the amplifier WA. Vhen the output of the repeater' amplilier WA is returned to normal level the balance between resistances 2T and 28 is restored and the galvanometer 23 automatically stops the rotation ot shaflt QQ.

Assuming, however, that the resistance elements of the potentiometer 2O moved along the adjusting taps until the maximum resistance is reached and still the output level ol amplifier NA is below normal, it Will be seen that it will then be impossible to set up a voltage balance between res f 27 and 2S and hence the shaft 22 iv'ill continue to rotate in a counter-clockwise direction. As the shaft 22 continues to rotate the resistance elements ot' potentiometer 2l will move along the potentiometer taps and introduce a loss between'the output el the pilot oscillator of P01 and the pilot tilter PF4. The voltage supplied to the rectifier bridge circuit Q6, which is bridged across the output circuit ot pilot oscillator' P01 between potentiometer 2l and pilot iilter Pl?, will be correspondingly decreased. and hence the direct current potential set up in resistance Q8 ot the rectifier bridge will be decreased until a voltage balance is established between resistance 2S of rectilier bridge circuit 26 and resistance 27 ol rectitier bridge circuit Vhen such a. voltage balance has been established the galvanometer Q53 automatically stops the rotation ol' shaft It will be noted from the above description that the output ot the pilot oscillator P0l has been decreased. and this decrease in output is exactly equal to the difference between the desired operating level and the level actually obtained in the output ot the amlVhen the line section lV decreases in at-` tenuation the output ot amplilier YA will ii crease correspondinglyv and the shaft 22 will be caused to rotate in a clockwise direction, and before it begins to decrease the gain or amplilier WA the output of the pilot oscillator P01 will be restored to normal.

lt will thus be seen that the system functions to automatically maintain a constant overall equivalent at all times regardless ot transn'iission changes on any line section.

The invention set forth herein is, of course, susceptible of various other modifi` cations and adaptations and therefore is not to be considered as limited in any way except as de lined by the scopeot the app-ended claims.

lli/That is claim-ed is:

l. A line divided into sections, repeater' ceeding said predetermined limits Jfor var i,l`r

ingl the gain characteristics ot a repeater at a repeater station on said line remote trom said line section. y

:2. In a system ot transmission regulation, a if'ariable conducting path, a pair of ter mmal stations and an intermediate repeater associated with said path, means at one terminal tor transn'iitting a control current over said lpath to said repeater, a separate source ot' control current at said repeater 'for transn'iitting a control current over said path to the other terminal, and means controlled by the control currents received at and transmitted lrom said repeater to indicate variations in the characteristics of said path.

3. in a system oitransmission regulation, a variable conducting pat-h, a pair ot terminal stations and an intermediate repeater associated ivith said path, means at one ter-4 minal to transmit a control current over said path to said repeater, means at said repeater to transmit a control current over said path to the other terminal, a control device at said repeater, and means for imlll() pressing potentials derived from the transmitted and received control currents at said repeater upon said device.

4i. In a system of transmission regulation, a variable conducting path, a pair oi terminal stations and an intermediate repeater associated with said path, means at one terminal to transmit a pilot current ver said path to said repeater', means including a source ot `pilot currentl at said repeater to transmit a pilot current over said path to the other terminal, means tor varying the pilot current transmitted trom said source at said repeater' under control oi pilot current received at said repeater trom the terminal and means controlled by the transmitted and received pilot currents at said repeater to adjust the transmission ot said conducting path.

In a system oi transmission rc elution, a conducting path ivhoiie trans ion is 'variable under diiiier-cntconditions, a. pair ot terminal stations and an intermediate repeater associated with said conducting path, means at one terminal to transmit a pilot current over said path to said repeater, means at said repeater to transmit a control current over said path to the other terminal, a current indicating,` device 'for said repeater, means to diiierentially in'iprcss potentials derived from the transmitted and received pilot currents at said repeater upon said device, and means controlled by said device to adjust the transmission et said conducting path.

6. In a system ot transmission regulation, a variable conducting` path, a pair et terminal stations and a repeater associated with said path, means to sinmitaneously transmit a plu `ality ot signals over said path by means of carrier currents of diiierent trequencies, means to impress a pilot current upon said path at one terminal station, means to receive said pilot current at said repeater, means to impress a second pilot current upon said path at said rep-eater, means to receive said second pilot current at the other terminal station, control apparatus at said repeater, means at said repeater to impress potentials derived from the transmitted and received pilot currents upon said control apparatus to adjust the transmission of said conducting;` path, and means associated with said control apparatus and operable under pr determined conditions to adjust the pilot current impressed upon said path at the repeater.

7. In a signaling' system in which a plurality ot signals are simultaneously transmitted over a variable conducting path by means of carrier currents ot different frequencies adapted to be amplified at a plurality ot repeaters, the method of regulating the transmission oi said signals which comprises subjecting a pilot current to substantially the same conditions of attenuation as those adecting the signaling frequencies transmitted between successive repeaters, utilizing,r the pilot current received at one repeater'` to compensato tor attenuation changes in said conducting path, and utilising the carrier current received at another repeater to compensate for such attenuation changes in said path as are not fully compensated for at said first mentioned repeater.

In a. signalingl system in which a plurality ot' signals are simultaneously transmitted over a variable conducting path by means ot carrier currents of dii'erent lirequencies adapted to be ampliiied at a plurality of repeaters, the method oi regulating` the transmission ot'V said signals which comprises subjecting a pilot current to substantially the same conditions otl attenuation as those affecting` the signaling` tre.- quencies transmit-ted between successive repeaters, producing an indication at one repeater by means ot' the pilot currentA received ther-eat, adjustingj the t'ansmission characteristics ot the conducting path in accordance with the indication produced, producing at another repeater by means of the pilot current received thereat an indication ot the attenuation changes in said path which are not fully compensated for at said iirst repeater, and adjusting the transmission characteristics of the conducting path in accordance With the indication produced at said second mentioned repeater.

9. In a. transmission system, a variable transmission line, means for transmitting waves over said line, a repeater associated with said line, means responsive to changes in an incoming Wave at said repeater to compensate for changes in the transmission characteristics of said line, a second repeater associated with said line, and means controlled from said first repeater to compensate at said second repeater for line changes which are not fully compensated tor at said first repeater.

l0. In a transmission system, a variable transmission line, means Ytor simultaneously transmitting a plurality ot' signals over said line by means otl carrier curr-ents oi different frequencies, a repeater associated with said line, means to transmit a pilot current over said line to said repeater, means at Said repeater' responsive to said pilot current to compensate for changes in the transmission characteristics ot said line, a second repeater associated with said line, means to transmit a pilot current from said {irst repeater to said second repeater, and means controlled by the pilot current received at said second repeater to compensate for line changes which are not fully compensated for at said tirst repeater.

11. In a transmission system, a variable conducting path, a path of terminal stations and an intermediate repeater associated with said path, means at one terminal for' transmitting a control curr-ent over said path to said repeater, means at said repeater to transmit a. control current over said path to the other terminal, means controlled by the transmitted and received control currents at said repeater to compensate for changes in the attenuation of said path, and mea-ns associated With said control means to adjust the pilot current impressed upon said path at said repeater in accordance with such attenuation changes as are not fully compensated for at said repeater.

12. In a transmission system comprising a line divided into successive sections, a pilot channel superposed on each successive section7 means at a terminus of each of. a plurality of sections controlled by changes in the pilot currentv received over the corresponding section to indica-te changes inthe transmission characteristics of the line within predetermined maximum limits, and means associated with the first means and rendered responsive by changes in such pilot current in excess of those corresponding to said maximum limits for controlling the pilot current transmitted over the succeed'- iugr section of said line.

In witness whereof, I hereunto subscribe my name this 25th day of November, A. D. 1924:.

JACOB S. JAMMER. 

